Abstract:The Danube Delta is a relatively young territory, formed about 14,000 years ago. It has quadruple status: Biosphere Reserve, Ramsar site, UNESCO World Heritage site, Natura 2000 site. Water and human activities are the most important factors influencing the flora of this area, including the penetration and spread of alien plants. The main goal of our research in this area was to inventory the alien plants and to identify those species which are invasive and potentially invasive in the natural and semi-natural ecosystems in order to propose measures for their prevention and mitigation. An inventory of these plants, conducted between 2009 and 2012 and based on bibliography and field research, comprises over 160 taxa. About half of them originated from America and less than a quarter of them from Asia. A relatively high number of species have unknown status in the Danube Delta; they were reported only from one or two localities and we did not record them during our extensive field work. In this category we also included some taxa of Xanthium without a very clear taxonomy. The taxa recorded as casual are usually ornamental plants escaped from cultivation; however among them there are some species which are known as invasive in other areas of Romania, as well as in Europe. There are 26 naturalised species, two of which established here over one hundred years ago. 37 invasive species were identified, many of them recorded in natural or seminatural places. In order to prevent and mitigate the spread of plants recognised as invasive, we propose the implementation of some measures such as providing relevant information to local communities and raising awareness about the damages caused by the alien species, and promoting further research on alien plant species in this protected area.

Symphyotrichum ciliatum (Ledeb.) G.L. Nesom is an adventive plant first reported in Romania in 1967, which has spread rapidly over the last few decades in Moldova, Muntenia and, more recently, in Transylvania. Although the species has been known for a while in Europe and Romania, there is no information about the anatomy of the vegetative organs of this invasive taxon. This paper presents a series of structural aspects of the vegetative body, of Symphyotrichum ciliatum collected from a sandy and salty substrate (Sacalin Island - Danube Delta). These demonstrate the ability of this plant to adapt to the environment and its capacity to achieve a wide spread. As such, although this plant is an annual species, the root and stem achieve secondary growth in their width, which offers robustness to the corm; the palisade tissue has an ecvifacial disposition which enhances efficiency in capturing light, especially on sandy soil that reflects light; there are aeriferous and aquiferous formations that ensure efficiency in adapting to a wet and relatively salty environment. These results are documented by original photographs of optical microscopy and a distribution map of the taxon in Romania as at 2011.